In a conventional liquid level detecting sensor for a fuel gauge, a bar joined to a float which is displaced in accordance with the height of the liquid level of fuel is rotated in accordance with the height of the liquid surface, and the height of the liquid level is detected on the basis of variation of a resistance value of a variable resistor provided to the movable shaft of the bar (for example, see Patent Document 1). The thus-constructed liquid level detecting sensor has a problem that the workability when the liquid level detecting sensor is secured to the fuel gauge insertion port is low because the bar joined to the float is larger than the diameter of the insertion port of the fuel gauge, and also a problem that metal powder may occur or such a trouble as short-circuit of a variable resistor may occur because there is some sliding portions in the fuel tank. In order to avoid these problems, a liquid level detecting sensor using an optical fiber is known. This liquid level detecting sensor is equipped with an optical fiber provided at a position where the height of the liquid level is desired to be detected, light emitting means for irradiating light a laser beam from one end portion of the optical fiber, and control means for determining a temperature distribution of the optical fiber on the basis of the intensity of back scattering light of the laser beam irradiated from the light emitting means and determining the height of the liquid level on the basis of the temperature distribution (see Patent Document 2, for example).
Furthermore, a sensor using an optical fiber for detecting the property of fuel is shown in Patent Document 3. The patent document 3 discloses a liquid property identifying sensor including a sensor probe with a tapered detector formed by subjecting an optical fiber to a fusing-drawing treatment over a predetermined length, a multiplexing optical coupler that is connected to the input terminal of the sensor probe and multiplexes light beams of at least two wavelengths so that the light beams of at least two wavelengths are incident to the input terminal, a demultiplexing optical coupler that is connected to the output terminal of the sensor probe and demultiplexes light of at least two wavelengths transmitted through the sensor probe, and light receiving means for detecting the amount of light of at least two wavelengths emitted from the output terminal of the demultiplexing optical coupler, the transmitted light amount of light incident from the input terminal being varied in accordance with the property of liquid in which the sensor probe is immersed.
[Patent Document 1]    JP-A-10-26552 (Page 5, FIG. 4)
[Patent Document 2]    JP-A-2004-294375 (Page 3, FIG. 1)
[Patent Document 3]    JP-A-6-58878 (Page 5, FIG. 1)